Neurally Adjusted Ventilatory Assist (NAVA) uses the electrical activity of the patient's diaphragm (EAdi) to trigger and cycle-off the pressure delivered to the patient's respiratory airways (Paw), as well as to adjust this pressure Paw in proportion to the electrical activity EAdi throughout patient's inspiration (Sinderby et al. [5]). The electrical activity EAdi of the patient's diaphragm is representative of the patient's respiratory drive and is influenced by facilitatory and inhibitory feedback loops that integrate information from mechano- and chemo-receptors as well as voluntary and behavioral inputs (Alla et al. [1]; Sinderby et al. [4]). With NAVA, the electrical activity EAdi of the patient's diaphragm simultaneously controls the diaphragm and the ventilator which hence conceptually acts as an external “respiratory muscle”. Studies in animals and healthy human volunteers show that NAVA efficiently unloads the respiratory muscles, prevents excessive lung distension (Sinderby et al, [4]; Beck et al. [2]; Allo at al. [1]) and improves patient-ventilator synchrony (Beck at al. [2]). NAVA is described in U.S. Pat. No. 5,820,560 (Sinderby at al.) entitled “Inspiratory Proportional Pressure Assist Ventilation Controlled by a Diaphragm Electromyographic Signal” and in U.S. Pat. No. 6,588,423 (Synderby) entitled “Method and Device Responsive to Myoelectrical Activity for Triggering Ventilatory Support”.
The electrical activity EAdi of the patient's diaphragm is derived from a linear array of electrodes such as 10 (FIG. 1) and a reference electrode (not shown) mounted on the distal end section of a naso-gastric feeding esophageal catheter 11. The esophageal catheter 11 consists of a multiple lumen esophageal catheter. The electrodes 10 can be formed by wrapping a wire at least one turn around the catheter 11 after having removed the insulation from the wire. The esophageal catheter 11 and the linear array of electrodes 10 are then coated with hydrophilic medical grade polyurethane (not shown), providing a conductive and slippery surface covering the electrodes 10. This slippery surface eases insertion of the esophageal catheter 11 through the mouth or a nostril and then through the esophagus until the linear array of electrodes reaches the patient's diaphragm and reduces friction between the catheter/electrodes and patient's mucosa. All the EAdi signals such as 12 are differentially recorded. More specifically, as depicted in FIG. 1, the signal 13 between the first and second electrodes (electrode pair 1), the signal 14 between the second and third electrodes (electrode pair 2), the signal 15 between the third and fourth electrodes (electrode pair 3), and so on (until electrode pair 7 in the example of FIG. 1) are recorded. As illustrated in FIG. 1, all the EAdi signals 12 can be summed to enable an ECG trigger to detect patient's ECG (see signal 16 at the top of FIG. 1 and U.S. Pat. No. 5,671,752 (Sinderby at al.) entitled “Diaphragm Electromyography Analysis Method and System”).
As shown in FIG. 2, a cross-correlation algorithm (see curve 20) is used to determine the most negatively correlated pairs of electrodes 10. The EAdi signals from these most negatively correlated pairs of electrodes 10 are assumed to represent the electrical activity EAdi of the patient's diaphragm when the linear array of electrodes 10 passes through the diaphragm and is substantially centered about the patient's diaphragm (see U.S. Pat. No. 6,584,347 (Sinderby) entitled “Disturbance-Free Electromyographic Probe”; Published US Patent Application 2004/0230110 A1 (Sinderby et al.) entitled “Control of Inter-Electrode Resistivity to Improve Quality of Measured Electrical Biological Signals”; Beck at al. [3]; and Sinderby et al. [6]).
In brief, the human crural diaphragm forms a few centimeter thick muscular tunnel around the esophagus, where the muscle fibers run mostly perpendicular to the esophageal catheter 11. The diaphragm around the esophagus defines an electrically active region (EARdi) during contractions. The linear array of electrodes 10 within the esophagus is oriented perpendicular to this region. As illustrated by the curve 20 of FIG. 2, EAdi signals measured simultaneously via pairs of electrodes 10 (among electrode pairs 1-7) positioned on the same side of the diaphragm have a correlation coefficient close to +1, whereas EAdi signals measured via pairs of electrodes 10 on opposite sides of the diaphragm have a correlation coefficient close to −1. Such cross-correlation analyses are performed between segments of non-processed differentially recorded EAdi signals obtained via the seven pairs 1-7 of electrodes 10. The most negative correlation coefficient between any two pairs of electrodes (between electrode pairs 3 and 5 in the example of FIG. 2) indicates that the respective EAdi signals are the most reversed in polarity. The electrode(s) 10 located between these two most negatively correlated pairs is(are) the electrode(s) closest to the center of the EARdi region.
In order to effectively detect the electrical activity EAdi of the patient's diaphragm, the array of electrodes on the distal end section of the esophageal catheter must be adequately positioned at the level of the diaphragm. This position will also allow the linear array of electrodes 10 to cover the inspiratory and expiratory displacement of the diaphragm.
However, a problem when detecting electrical activity EAdi of the patient's diaphragm is positioning of the catheter within the patient's oesophagus. To obtain proper EAdi signals some of the electrodes of the linear array should be placed above the diaphragm and some below the patient's diaphragm. There is a possibility that the esophageal catheter 11 will be inserted too far, or not be inserted far enough. In both cases the array of electrodes 10 mounted on the distal end section of the esophageal catheter 11 will detect either weak EAdi signals or even may not capture any signal at all. The esophageal catheter 11 may also capture myoelectrical signals from other muscles instead of, or in addition to, the EAdi signals from the patient's diaphragm. Hence, there is a need for an improved method and device for appropriately positioning a linear array of electrodes mounted on a distal end section of an esophageal catheter in a patient's respiratory airways at the level of the patient's diaphragm.